This invention relates to image pick-up tube targets and, more particularly, to an image pick-up tube target capable of effectively suppressing high-light sticking of the photoconductive film with respect to high brightness objects.
An image pick-up tube using the photoconductive film mainly containing Se, As and Te as target, as disclosed in U.S. Pat. No. 4,007,395, for example, has a drawback, so-called high-light sticking, in which a long trail follows a moving high brightness object being picked up and when the object is stationary, the trail persists for a long time. In order to overcome the above drawbacks, a proposal has been made by the inventors, by which LiF is doped in the Se-As-Te film to obtain very satisfactory effects.
High-light sticking can be resolved into an afterimage component and a sticking component. An afterimage component is a result of a phenomenon which occurs when the background of a high-light is dark and as the high-light is moved, it is observed as a tailing and is measured in terms of time till the time the tailing disappears. A sticking component then is due to a phenomenon caused when a high-light is imaged when the background is light. It is a phenomenon similar to printing as the term is used in the field of photography. Compared with a high-light afterimage, a high-light sticking is a phenomenon of long time constant and is expressed in terms of percentage in relation to a reference signal level.
However, if fluoride is doped in a Se-As-Te film in such an image pick-up tube target, the rising of current in the voltage-current characteristic of the photoconductive film is deteriorated and consequently, sufficient image signal current cannot be taken out, so that the image pick-up tube cannot be operated with practically satisfactory performances.